Determination of grain boundary chemistry in functional materials with atomistic precision using a novel and cheap X-ray methodology in a scanning electron microscope

About the Project

Intergranular films due to segregation of dopants, internal oxidation or other chemical reactions at interfaces govern the properties of many polycrystalline materials: segregation can lead to brittle fracture of steel or copper pipes. It influences the wear properties of hard ceramics. Internal oxidation of pores can weaken sintered aluminium alloys. Cation segregation in oxide ceramics determines their electrical resistance important for their use as varistors. Gate oxide thickness and chemistry determine the switching behaviour of field-effect transistors used in microelectronics. It is, however, often quite difficult to learn anything about such intergranular films within materials unless they break to reveal their inner surfaces which can then be accessed by surface chemical methods.

Analytical electron microscopy is one of the key tools with sufficient spatial resolution and chemical sensitivity to study such buried interfaces and thin films. In this project we will use a novel methodology of scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDX), which will generate enhanced chemical sensitivity and improved precision for investigating atomically thin films.

We will develop a new and cheap methodology that can determine with atomic precision the chemistry of thin films segregated to grain boundaries in functional ceramics, quantify internal oxidation in metallic alloys and measure gate oxide thicknesses in field-effect transistors. The project proposal is to test and qualify a new method based on linear regression analysis applied to energy-dispersive X-ray (EDX) maps recorded in scanning electron microscopy (SEM). Similar methods were originally developed by the supervisor for transmission electron microscopy (TEM) and later adopted for scanning TEM (STEM) where they yielded some impressive results on doped inversion domain boundaries in zinc oxide. This methodology is now to be transferred to scanning electron microscopy (SEM) where it is much easier and cheaper to implement.

We will produce two prototype samples to model near-perfect interfaces between atomically thin intergranular films based on a series of controlled metal thin film deposition experiments on silicon and germanium wafers in the university's cleanroom. These can serve as well-defined test structures for intergranular films between materials of light and medium densities, respectively, for investigation by SEM-EDX.

This project

1. advances a new methodology based on existing SEM-EDX hardware that is both widely available in engineering and technology and much quicker and cheaper to implement than TEM-EDX based experimentation, advancing chemical sensitivity and accuracy levels for different materials by an order of magnitude over comparable existing analysis methods.
2. is a prerequisite for being able to control grain boundary segregation of cations, internal oxidation or gate oxide thicknesses more accurately, thereby increasing manufacturing yield by improving the (mechanical or electronic) properties of manufactured devices and at the same time improving resource efficiency by reducing materials waste.